The present invention relates, in general, to ionization detectors. More particularly, this invention is concerned with an improved form of radiation source for use in ionization detectors. These detectors are usually used for sensing particles of combustion or aerosols found in the smoke generated from a fire.
In my copending application Ser. No. 593,704, there are discussed some of the advantages and disadvantages associated with the use of an alpha source of radiation for use in ionization detectors. For one thing, alpha sources are not as safe to use as beta sources of radiation. Also, when an alpha source is used, care must be taken to insure that the active sensing chamber is not saturated by the ionic current. To control the current, some known devices have main electrodes that are adjustable relative to the distance therebetween. When an alpha source is used and when the number of ion-pairs formed is excessive, the ionization current stabilizes and the device essentially functions as a constant current generator. In a plot of ion-pairs formed versus ionization current, the curve is fairly linear at low ionization currents and as more ion-pairs are formed the ionization current saturates. Some prior art patents such as U.S. Pat. No. 3,233,100 examine this effect and provide a special construction to maintain the ionization current at a low value. However, this provides a more complex structure. On the other hand if one uses a beta source, a far smaller number of ion-pairs are produced.
However, there is still a further disadvantage to using a relatively small radiation source even though it is a beta source. With a small source the space within the ionization chamber is not efficiently utilized due to the influence of the radiation becoming diffused away from the source. It therefore follows that the sensitivity to particles of combustion or aerosols varies depending upon the zone within the chamber. It has thus been found that it is desirable to have a homogeneous area of influence throughout the ionization chamber. Furthermore, when one desires to use a source of beta particles, it is especially important to have a homogeneous field of influence in the chamber.
Accordingly, one object of the present invention is to provide a radiation source, preferably a source of beta particles, that provides a homogeneous field within the radiation chamber.
Another object of the present invention is to provide a source of radiation for use in an ionization chamber and preferably a source of beta particles wherein the source is constructed to provide an increase ionic current and concurrently provide a homogeneous field of influence within the chamber.